Knockout & Jasmine not mixing well

Task.js

var TASKLISTMODULE = (function(module) {
  module.Task = function (name, owner, duedate, status) {
    var self = this;

    self.name = ko.observable(name || '');
    self.owner = ko.observable(owner || '');
    self.duedate = ko.observable(duedate);

    self.status = status || 'unassigned';

    self.duedate_readable = function() {
      var formatting = 'm/d/Y H:i:s';

      var dd = self.duedate();
      if( system.istoday(dd) ) { formatting = 'H:i:s'; }

      return dd.format(formatting);
    };

    var statuses = {
      'unassigned': 'Unassigned'
      , 'assigned': 'Assigned'
      , 'in_progress': 'In Progress'
      , 'completed': 'Completed' };


    self.status_readable = function() {

      return statuses[self.status];
    };
  };

  return module;
}(TASKLISTMODULE || {}));

TaskSpecs.js

describe('TaskModel', function() {
  beforeEach(function() {
    this.task = new TASKLISTMODULE.Task();
  });
  it('should default name to empty', function() {
    expect(this.task.name()).toEqual('');
  });

  it('should default owner to empty', function() {
    expect(this.task.owner()).toEqual('');
  });

  it('should default Due Date to null', function() {
    expect(this.task.due).toEqual();
  });

  it('should default Status to unassigned', function() {
    expect(this.task.status).toEqual('unassigned');
  });

  it('should hide date for duedate_readble when date is today', function() {
    // year, month, day, hours, minutes, seconds, milliseconds
    system.today = function() { return new Date(2012, 0, 2); };

    this.task.duedate( new Date(2012, 0, 2, 14,0,0) );

    expect(this.task.duedate_readable()).toEqual('14:00:00');
  });

  it('should show date for duedate_readble when date is not today', function() {
    // year, month, day, hours, minutes, seconds, milliseconds
    system.today = function() { return new Date(2012, 0, 1); };

    this.task.duedate( new Date(2012, 0, 2, 14,0,0) );

    expect(this.task.duedate_readable()).toEqual('01/02/2012 14:00:00');
  });

});